1. Field of the Invention
The present invention relates to a method and apparatus for controlling concentration of a water treatment chemical (a water treating chemical agent or a chemical additive to a water system) within water of a recirculating water system such as a cooling water system.
2. Description of the Related Art
Process water such as industrial water serves a significant role in many industries. Process water is often used in a recirculating water system, which may typically be a boiler water system or an open or closed recirculating cooling water system. For treating water used in such a recirculating water system, various water treatment chemicals (chemical agents) are employed to prevent problems such as corrosion, scale, and slime attributable to by water. In general, water treating chemical agents may include a corrosion inhibitor, dispersant, scale inhibitor, antimicrobial, and slime control agent (biofouling inhibitor).
In order to perform appropriate water treatment by making use of the effectiveness of these various chemical agents and to allow the effectiveness to sustain, it is necessary to precisely determine the concentration of the chemical agents at given positions and times, so as to execute appropriate control of the concentration.
However, measurement of concentration within water to be treated is impossible for some chemicals, depending on the type of the chemical. Further, although the measurement may be possible by colorimetry, turbidimetry, or other quantitative analysis methods, procedures for these methods may be complicated or require a long time such that the methods are impractical for plant operation control. Accordingly, when using a water treatment chemical for which concentration measurement is impossible or difficult, concentration control may be performed using, as a tracer, a substance that allows easy concentration measurement. By using a tracer substance, concentration of a chemical agent within water to be treated can be indirectly but quickly measured even when concentration measurement of the chemical agent itself is impossible or difficult. Conventional methods for measuring the concentration of a water treatment chemical using a tracer substance may be categorized by the type of tracer substance. Major examples of the tracer methods are fluorescent tracer method, pigment tracer method, bromine tracer method, iodine tracer method, potassium tracer method, and lithium tracer method.
It is generally desired that a substance used as a tracer fulfill many conditions as listed below. (1) The substance does not exist in process service water such as industrial water, or the amount of its presence in water is a negligibly trace amount. (2) The substance is chemically stable. (3) The substance is not easily decomposed by microorganisms. (4) The substance is substantially harmless from a pollution prevention perspective. (5) The substance does not generate insoluble matters or scale by reacting with salts dissolved in process water such as industrial water. (6) The substance does not corrode the metal materials of the piping and other components of the water system. (7) The quantitative analysis of the substance is possible without being interfered by dissolved salts. (8) The quantitative analysis can be performed precisely and quickly.
Among the conventional methods for measuring concentration of a water treatment chemical, a fluorescent tracer method is disclosed in Japanese Patent Laid-Open Publication No. Hei 2-115697, while a pigment tracer method is disclosed in Japanese Patent Laid-Open Publication No. 2001-334255. According to these methods, many of the organic substances employed as the tracer tend to degenerate by the influence of light, heat, oxidants, pH, and the like within the recirculated water such as boiler water or cooling water (especially cooling water in an open recirculating system). Consequently, measurements as a function of time of the tracer concentration tends to a decrease irrespective of the water treatment chemical concentration, resulting in the problem that the tracer concentration cannot be correctly converted into the water treatment chemical concentration. Further, the recirculated water including water treatment chemicals in addition to the tracer may contain, at varying degrees, a light-absorbing substance or a fluorescent substance generated due to fouling such as sludge. As a result, measurements must be made taking into account the interferences from those matters.
As another conventional method for measuring concentration of a water treatment chemical, a method using bromide ion or iodide ion as the tracer is disclosed in Japanese Patent Laid-Open Publication No. Hei 4-296651. This publication describes using titration and ion electrode method for quick and simple measurement of bromine or iodine, and using ion chromatography when requiring high sensitivity or simultaneous measurement of other anions. However, procedures for titration are complicated, and it would be very difficult to automate titration analysis. Real-time concentration control by titration is therefore impossible at the present. Further, bromide ion electrode method and iodide ion electrode method are disadvantageous in their poor selectivity with respect to other anions and the susceptibility to interferences from impurities contained in the cooling water. Ion chromatography requires a large-scale instrument, as well as a long time from the point when sample water is collected to the point when a measurement result can be obtained, so that real-time concentration control is impossible.
A potassium tracer method is disclosed in Japanese Patent Laid-Open Publication No. Hei 4-296652. This publication suggests use of an ion electrode method as the quick and simple measurement method. However, a certain amount of potassium ion is originally included in the raw water of the cooling water. The potassium tracer method is therefore susceptible to influences from fluctuations in the concentration of potassium included in the raw water supplied as the make-up water for the cooling water.
A lithium tracer method is disclosed in Japanese Patent Laid-Open Publication No. Sho 51-111388. This method requires performing atomic absorption spectrometry, which involves using a large-scale instrument. Further, because a long time is required from the point when sample water is collected to the point when a measurement result can be obtained, real-time concentration control is impossible.